PA6-CF or PC for an under-bonnet part?

The question to ask first: what temperature, and for how long

Before the material comes the real use: the temperature the part lives at, and how long it stays there. A peak of a few minutes at start-up and continuous heat all day long do not call for the same thing. Tell us the real temperature and its duration: nine times out of ten, that is enough to decide.

The catalogue benchmarks are simple. PA6-CF withstands 120 °C continuously, PC 110 to 120 °C. Above that, you change family: PPA-CF goes up to 180 °C continuously, PPS-CF up to 200 °C. And if the part is mainly exposed to sun and weather rather than to engine heat, ASA, which takes 90 °C and does not yellow, is often the right answer, for far less money.

PA6-CF: stiff, light, no creep

Carbon-fibre nylon is the material for parts that stay under permanent stress without deforming over time: brackets carrying a continuous load, clips and tabs that must keep their spring after several assemblies, jigs, on-board parts. It is stiff, light, and its creep is very low: where PETG would eventually go slack, it stays straight.

Its limits are known. It costs three times the price of PETG, it only comes in black, and its surface is matt and slightly rough: it is not a cosmetic material. Above all, PA6 nylon absorbs moisture, up to 3 % of its weight, and then loses stiffness. Under a bonnet the atmosphere is sometimes dry and scorching, sometimes damp: if the part must hold its dimension whatever happens, PAHT-CF, based on PA12, caps at around 0.3 % absorption and takes 130 °C. PET-CF, just as stiff, resists workshop oils and greases better, but it is brittle: it gives no warning before breaking.

PC: impacts and temperature

Polycarbonate is the material that takes over when the 90 °C of ABS is no longer enough: machine guards, covers close to a heat source, parts that take repeated impacts, brackets and guides for ovens and drying tunnels. Where carbon nylon is stiff, PC is tough: it absorbs the shock instead of transmitting it.

It is tricky to print, so it needs more machine time and costs more than a PETG; it is moisture-sensitive and must be dried before printing; and its transparent grade is never optically clear in FDM, the layer lines scatter light. When a fire-behaviour requirement is added, PC-FR, rated UL94 V-0, takes its place: the manufacturer's datasheet ships with the order, bearing in mind that the rating applies to the material, not to the finished part.

How to decide

The rule we apply fits on one line: if the stress is mainly mechanical, a permanent load, a part that must stay straight, it is PA6-CF; if it is mainly thermal or made of impacts, it is PC. When both add up above 130 °C, it is PPA-CF, the highest mechanical strength in the catalogue, at 180 °C continuously.

And we say no to over-specification. Many requests come in for PA6-CF when PETG is more than enough; between PPS-CF and PC, nine times out of ten PC is enough and costs four times less. An over-specified part has never won anyone's loyalty: it costs more, it is harder to print, and it does not do the job any better.

Dimensions, shrinkage and series

We hold ±0.3 mm as standard, and ±0.15 mm on dimensions identified in advance, negotiated one by one; beyond that the part would need machining, which we do not do. On these materials shrinkage matters more: PPA-CF, for instance, is reserved for parts of moderate size, because the chamber tops out at 65 °C and shrinkage pulls on large parts.

That is why, above 50 parts, a sample is approved in writing before the run, and systematically offered below: you fit the part, you check the real fit at the real temperature, and production starts after your go-ahead. Allow 2 to 3 working days for a prototype, 5 to 8 days for a run of 50 to 250 parts.

What to send us

An STL, OBJ or 3MF file for direct printing, a STEP if design rework is needed; without a file, a dimensioned 2D drawing, a photo with a rule or the original part are enough, we model it. Add the temperature, its duration, the nature of the load and the environment (oil, moisture, outdoors): that is what decides the material.

The instant quote prices your file in both materials, and the firm quotation goes out within 24 working hours. The manufacturer's datasheets are supplied on request; we do not issue part certification, that is not our trade.

On the same subject

• PA6-CF datasheet
• PC datasheet
• PPA-CF datasheet
• Which material for a part that runs hot?

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